This week, I'm travelling from Rugby to Euston and back with Virgin
every day. I will then need to make a daily return trip from Euston
underground to Westminster underground.

Would it be cheaper to buy 5 x daily zone 1 return tickets or an Oyster
card? I've no idea how Oyster works - can I buy it at Euston tomorrow
morning and do I simply put money onto it, i.e. pre-pay? Does this
method work out cheaper than buying ordinary single/returns?

Thanks